The present invention relates to a carbon dioxide sensor for measuring concentration, partial pressure, or presence/absence of carbon dioxide in a gas expired through nostrils or a mouth of a living body.
In general, when the concentration of carbon dioxide contained in an expiration gas expired from a living body is optically measured, the gas is led through a cylindrical airway adapter. An infrared ray is radiated onto the expired gas from a light-emitting element. The amount of light that remains after some of the light has been absorbed by the carbon dioxide contained in the expiration gas is detected by a light-receiving element, thus measuring the concentration of carbon dioxide.
FIG. 16 shows such an apparatus for measuring the concentration of carbon dioxide. In this apparatus, one end 101a of an airway adapter 101, which is formed into a substantially cylindrical shape and through which a respiration gas passes, is to be connected to a tube inserted into a trachea of a patient. Another end 101b is to be connected to a Y piece of a respiratory circuit, such as a respirator. An intermediate portion of the airway adapter 101 has a rectangular cross-sectional shape. Circular windows 101c, 101d are formed in respective, opposing surfaces of the intermediate portion such that the windows are concentrically aligned with each other.
A sensor body 102 is formed into a substantially-rectangular shape, and a notch is formed in an intermediate portion of the sensor body 102. The intermediate portion of the airway adapter 101 is to be detachably fitted with the notch. Two opposing surfaces defining the notch are in contact with the windows 101c, 101d of the airway adapter 101. A light-emitting element 103 is disposed on one side with reference to the notch.
An optical filter 104 for absorbing only light having a wavelength to be absorbed by carbon dioxide and a light-receiving element 105 are disposed on the side opposite the light-emitting element 103 with reference to the notch. The light-emitting element 103 and the light-receiving element 105 are connected to a monitor 107 via a lead wire 106.
In the apparatus having the foregoing configuration, the light emitted from the light-emitting element 103 enters the light-receiving element 105 by way of the window 101c, the respiration gas in the airway adapter 101, the window 101d, and the filter 104. The light-receiving element 105 detects the amount of light after some amount of the light has been reduced in accordance with the concentration of carbon dioxide. A signal output from the light-receiving element 105 is input to the monitor 107, where the concentration of carbon dioxide is displayed.
Another known apparatus has a structure in which a sampling tube is connected to a sensor body disposed in a monitor.
In such an apparatus, one end of the sampling tube which introduces a portion of a respiration gas is connected to an airway adapter through which the respiration gas passes. The other end of the sampling tube is connected to the monitor. A pump is disposed in the monitor to lead the introduced respiration gas to the sensor body disposed in the monitor.
Moreover, as shown in FIG. 17, an apparatus capable of measuring the concentration of carbon dioxide in an oral expiration gas as well as the concentration of carbon dioxide in a nasal expiration gas is known (see, e.g., U.S. Pat. No. 5,046,491).
This apparatus is provided with a respiration gas collector 110 having: a nasal cannula 111 for collecting a nasal respiration gas; an outwardly-convex mouth guide 113 for collecting an oral respiration gas; an oral respiration gas collector 114 which is disposed in the mouth guide 113 and collects an oral respiration gas; and a joint stem 112 which is connected at one end thereof to an external upper portion of the mouth guide 113 and at the other end thereof to the nasal cannula 111.
However, the respiration gas collector 110 involves a large number of components, because the joint stem 112 is constituted of separate members. Further, the joint stem 112 must be attached to two points; that is, the mouth guide 113 and the nasal cannula 111. This entails consumption of man-hours and, by extension, costs.
Further, in the respiration gas collector 110, the oral respiration gas collector 114 is disposed in the mouth guide 113 in order to cause a respiration gas to flow through an airway passage provided in the upper portion of the mouth guide 113. Hence, the oral respiration gas collector 114 exerts gas flow resistance, which inhibits efficient flow of the oral expiration gas through the airway passage.
In a case where oxygen is also supplied in conjunction with collection of the respiration gas, an oxygen supply tube is also attached to the patient. In such a case, prongs are inserted into nostrils. Alternatively, even in a case where an oxygen supply tube which does not entail insertion of the prongs into the nostrils, the prongs are oriented so that oxygen supplied by way of the prongs is injected directly toward the nostrils, which induces a problem of abrupt drying of the nostrils, causing the patient discomfort.